I own a Green Iguana which takes a lot of care and a huge amount of understanding.
I have spent countless hours getting her set up correct and in doing so, I have also tried many Thermometers & Humidity devices. I personally find it best to avoid digital because they are not always accurate.
The Sungrow Boro Analog Dual Thermometer & Humidity Gauge is very accurate.
This is a vital bit of equipment which every Lizard owner should have.